In many fuel injectors, a simple spring biased needle check is used to open and close the nozzle outlet. The needle valve member typically includes at least one lifting hydraulic surface that is acted upon by fuel pressure. A compression spring is positioned to bias the needle toward its closed position. When fuel pressure rises above a valve opening pressure sufficient to overcome the spring, the needle valve member lifts to open the nozzle outlet to commence an injection event. Each injection event ends when fuel pressure drops below a pressure necessary to keep the needle valve open against the action of the biasing spring. When this occurs, the spring pushes the needle valve member downward to its closed position to end the injection event.
An improvement on the simple spring biased needle check is described in U.S. Pat. No. 5,429,309 to Stockner, which improvement is more commonly known as a trapped volume nozzle. In a typical fuel injector employing a trapped volume nozzle, the compression biasing spring is positioned in a closed volume space. During an injection event, high pressure fuel migrates up the outer surface of the needle valve member and into the trapped volume. This raises pressure in the trapped volume relatively high, and sometimes in excess of 20 MPa. The purpose of the trapped volume is to increase the speed at which the needle valve member moves to its closed position at the end of an injection event. Those skilled in the art are well aware that in most instances it is desirable to make an injection event end as abruptly as possible in order to decrease undesirable noise and improve emissions from the engine. The trapped volume nozzle achieves this goal by having the needle valve member pushed toward its closed position at the end of an injection event not only by the force of the biasing spring but also by a hydraulic force due to the pressure in the trapped volume acting on one end of the needle valve member.
Although the concept of a trapped volume nozzle has proved sound in hastening the closure rate of the needle valve member, an undesirable side effect has been observed. In some instances, the relatively high pressure developed in the trapped volume during an injection event is unable to decay to a relatively low pressure between injection events. This has the effect of raising the valve opening pressure for a subsequent injection event since the needle valve member is being held closed by hydraulic pressure in addition to the force of the compression biasing spring. Not only is the valve opening pressure for subsequent injection events raised, but the pressure decay has a tendency to vary significantly between individual injectors so that the valve opening pressure is difficult to predict. The end result of this effect is that the injectors tend to inject less fuel than expected, and the amount injected is somewhat unpredictable.
The present invention is directed to overcoming these and other problems associated with fuel injectors employing trapped volume nozzle technology.